Packings, O-rings, and the like to be used around fuel tanks are each required to possess fuel oil resistance, ozone resistance, compression set, and low-temperature performance, as essential functions.
To exhibit such functions, rubber materials are used, each adopting a high nitrile polymer.
It is typical for such a sealing rubber material, to blend therein carbon black, zinc oxide, stearic acid, ozone-resistant wax, age resister, plasticizer, sulfur, and vulcanization accelerator.
However, it has been pointed out that, in case of adopting a product having the above components blended therein, those components extracted from the rubber by a fuel oil, particularly by an alcohol-containing fuel, are insolubilized, in a manner to possibly cause clogging of a fuel filter, thereby adversely affecting an internal combustion engine.
As a countermeasure thereto, it is conceivable to adopt a fluororubber as a sealing rubber material around a fuel tank. However, adoption of a fluororubber results in that a cost required for the sealing rubber material is increased to 10 times or more, thereby leading to a situation that inexpensive NBR compositions are highly demanded.
The present inventors have conducted an investigation of a reason of insolubilized substance generation, thereby resultingly confirmed that the reason resides in a wax component(s) for providing ozone resistance. Further, the present inventors have confirmed that stearic acid, which is typically blended as a processing aid, is also turned into an insolubilized substance.
However, in case that a wax component(s) is/are excluded from a blending formulation as a countermeasure to insolubilized substance generation, the ozone resistance, which is one of the important functions, is lacked; and in case that stearic acid is excluded from a blending formulation, the resultant rubber compound is caused to be deteriorated in ejectability from a kneader (pressure kneader) to be used when kneading the rubber compound; so that such exclusion leads to failure of establishment in terms of function and production.    Patent Document 1: JP-A-S63-256680    Patent Document 2: JP-A-2000-95899    Patent Document 3: JP-A-H11-304058    Patent Document 4: JP-A-2001-172433    Patent Document 5: JP-A-2006-22220